The present disclosure relates to a circuit protection device for protecting electrical circuits against excessive voltage and current conditions. In particular, the disclosure relates to an integrated circuit protection device having a positive temperature coefficient (PTC) current limiter and voltage suppressor within a single package.
Many electrical and electronic circuits require overcurrent protection and overvoltage protection. FIG. 1 shows a circuit diagram of a typical circuit incorporating overcurrent and overvoltage protection. Generally, the overcurrent and overvoltage protection is obtained through at least two discrete devices. Each device provides protection for a specific application. For example, a discrete PTC current limiter 10 provides protection during overcurrent situations. In addition, a discrete semiconductor device 12 (e.g., a voltage suppressor) provides protection during excessive voltages. The two discrete devices are interconnected through printed circuit board wiring. Consequently, valuable space of the printed circuit board is utilized by the footprint of each discrete component.
It is well known that the resistivity of many conductive materials change with temperature. Resistivity of a positive temperature coefficient (“PTC”) material increases as the temperature of the material increases. Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers generally include polyolefins such as polyethylene, polypropylene and ethylene/propylene copolymers. Certain doped ceramics such as barium titanate also exhibit PTC behavior.
At temperatures below a certain value, i.e., the critical or switching temperature, the PTC material exhibits a relatively low, constant resistivity. However, as the temperature of the PTC material increases beyond this point, the resistivity sharply increases with only a slight increase in temperature.
Electrical devices employing polymer and ceramic material exhibiting PTC behavior have been used as overcurrent protection in electrical circuits. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device due to I2R heating remains below the critical or switching temperature of the PTC device. The device is said to be in an equilibrium state (i.e., the rate at which heat is generated by I2R heating is equal to or less than the rate at which the device is able to lose heat to its surroundings).
If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases and the temperature of the PTC device (due to I2R heating) rises rapidly to its critical temperature. At this point, a great deal of power is dissipated in the PTC device and the PTC device becomes unstable (i.e., the rate at which the device generates heat is greater than the rate at which the device can lose heat to its surroundings). The power dissipation only occurs for a short period of time (i.e., a fraction of a second), however, because the increased power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device becomes so high that the current in the circuit is limited to a relatively low value. This new current value is enough to maintain the PTC device at a new, high temperature/high resistance equilibrium point, but will not damage the electrical circuit components. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, relatively low value when the PTC device is heated to its critical temperature range.
Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge), the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Overvoltages can be protected by the incorporation of semiconductor devices having voltage recovery characteristics like zener diodes or thyristors depending on the needs of the application.
Designing circuits having discrete electrical protection devices has a number of disadvantages. As mentioned previously, valuable space on the printed circuit board is occupied by two separate components. Printed circuit board designers are always looking for ways to reduce the footprint of components in an effort to reduce the circuit board space needed. Thus, it is desirable to reduce the size of integrated overcurrent and overvoltage protection devices.
Moreover, electrical coordination problems arise with the discrete devices making it difficult to assure coordination between the voltage suppressor protecting against overvoltage conditions and the thermal protector protecting during overcurrent conditions. Coordination between devices is important to eliminate damage to a circuit and its components in any overcurrent or overvoltage condition. One reason that makes coordination between the discrete devices difficult is because discrete devices are often times made by different manufacturers. As such, the burden is placed on the circuit board designer to coordinate the discrete devices together in order to obtain the proper overcurrent and overvoltage protection of the circuit and its components. Determining proper electrical coordination between the devices is not a simple procedure. It takes time to evaluate criteria of each device (e.g., I2t curves, etc.) to make sure that protection against excessive voltages and currents will result. In addition, specification tolerances for the discrete devices of different manufacturers may vary resulting in poor coordination between the discrete devices even when a proper evaluation is conducted by the designer.
Another concern with existing discrete devices is that the discrete devices may not be fast enough during overvoltage conditions for certain applications. To this extent, sensitive circuitry can easily be damaged.